Conventionally, the use of cholesteric liquid crystal layers for identifying cards and security notes has been known. Cholesteric liquid crystals normally have a layered structure, and the axial directions of the molecules in each layer are parallel to each other as well to the plane of each layer. Each layer is slightly twisted relative to the adjacent layer so that a three-dimensional spiral structure is produced. This structure demonstrates the property to selectively reflect a circularly polarized light having a wavelength of .lambda. which is given by .lambda.=n-p where p is the depth of the layers for this axial direction to turn 360 degrees or the pitch, and n is the average index of refraction of each layer. Therefore, if the direction of the liquid crystals in each layer turns counter-clockwise with respect to the incident light, the left-handed circularly polarized component of the incident light having the wavelength of .lambda. is reflected while the right-handed circularly polarized component passes through. The light having any other wavelength passes through. For instance, when a cholesteric liquid crystal material having a property to reflect red light having the wavelength of .lambda..sub.R is placed on a material which absorbs light in the visible range, and a random light such as sunlight is radiated thereon, the transmitted light is all absorbed, and only a left-handed circularly polarized light having the wavelength of .lambda..sub.R is reflected.
For instance, Japanese patent laid-open publication (kokai) No. 4-144796 discloses a system in which random light is radiated upon a cholesteric liquid crystal layer, and the reflected circularly polarized light is passed through a band pass filter and a quarter-wave plate to convert the incident light into a linearly polarized light. The linearly polarized light is divided by a beam splitter, and a right-handed circularly polarized light or a left-handed circularly polarized light is detected by using a suitable polarizing plate.
However, when reflected light is used for identification purpose, the surface contamination and/or irregular reflection from the background may cause noises which are significant enough to impair the reliability of the system. Also, the reliance on the simple use of a liquid crystal layer may not be effective enough because duplication or forgery is relatively easy. To individually detect a right-handed or left-handed circularly polarized light, an expensive beam splitter is required. This leads to an increase in the number of necessary components, in the size of the system, and in the overall cost.
It has also been proposed to affix a hologram on the surface of an object and to identify the authenticity of the object by visually identifying it. It has also been proposed, to eliminate the possible uncertainty associated with visual identification, to use a hologram or diffraction grating having a specific diffractive property, impinge a light beam having a prescribed wavelength upon the hologram, and determine the authenticity of the object by comparing the intensity of the light diffracted onto a prescribed position with the intensity of the light obtained at a different position,
However, due to the recent popularization of the technology of preparing hologram, the hologram technology has become so readily available that illicit duplication of hologram which is hardly distinguishable from an authentic hologram can now be made without any significant difficulty. In other words, the hologram has become less effective in discouraging illicit duplication. A light beam diffracted by a hologram or diffraction grating is typically detected by comparing its intensity with the intensity of a light beam obtained elsewhere and determining if the difference is greater than a prescribed threshold level or not. However, because of the need for an additional light receiving unit to be placed at a position other than that for the diffracted light beam, an increase in both size and cost was unavoidable. Also, any irregular reflection and/or insufficient reflection due to surface contamination could cause detection errors.
Other technologies for preventing forgery are known, but are so costly that they are not suitable for use on common commercial goods. Thus, there is a need for a novel technology for preventing forgery.